A liquid crystal display (LCD) includes a liquid crystal alignment film, and the liquid crystal alignment film is mainly made of polymer materials. The liquid crystal alignment film plays a role of a director in aligning liquid crystal molecules. When the liquid crystal molecules are moved by the influence of an electric field to display an image, the liquid crystal alignment film allows the liquid crystal molecules to be oriented in a predetermined direction. Generally, it is necessary to uniformly align the liquid crystal molecules in order to provide uniform brightness and a high contrast ratio to the LCD.
The conventional method of aligning the liquid crystal includes coating a polymer membrane such as a polyimide on a substrate made of a material such as glass, and rubbing the surface of the substrate with a fiber such as nylon or polyester in a certain direction. However, the rubbing method may cause serious problems while fabricating a liquid crystal panel due to fine dust or electrostatic discharge (ESD) that may be generated while rubbing the polymer membrane with the fiber.
In order to solve the problems of the rubbing method, a photo-radiation method has recently been researched to induce anisotropy to the polymer membrane by irradiating light on the membrane so as to align the liquid crystal molecules.
Polymers having diamine derived functional groups and photo-functional groups such as azobenzene, cumarine, chalcone, and cinnamate have been used as polymer membrane materials for photo-alignment methods. Such polymers are anisotropically photo-isomerized or photo-cross-linked when irradiated with polarized light, so as to provide anisotropy to the surface so that it can induce the liquid crystal molecules to align in a certain direction. The process of fabricating the diamine containing the photo-functional group, however, can be very complicated and thus is typically not cost effective.